update:demo3

com/study/demo3/EBook.java

@@ -0,0 +1,30 @@
+package com.study.demo3;
+public class EBook extends Book {
+    private double fileSize;
+
+   
+    public EBook() {
+        super();
+    }
+
+   
+   
+    public EBook(String isbn, String title, double fileSize) {
+        super(isbn, title);
+        this.fileSize = fileSize;
+    }
+
+   
+    public double getFileSize() {
+        return fileSize;
+    }
+
+    public void setFileSize(double fileSize) {
+        this.fileSize = fileSize;
+    }
+
+    @Override
+    public String toString() {
+        return "EBook [ISBN=" + isbn + ", Title=" + title + ", FileSize=" + fileSize + "MB, Available=" + available + "]";
+    }
+}